1. Field of the Invention
This invention relates to algae-retardant roofing granules having algicidal properties. More particularly, the invention relates to a two-coat product having: an inner coat containing a slow-release copper or bimetallic copper/zinc algaecides; and an outer coat having copper or bimetallic copper/zinc algaecides containing gas-forming compounds which form a network of micro voids to increase porosity and thereby facilitate leaching of the algaecides to enhance their algicidal properties.
2. Reported Development
Roofing granules, both natural and artificially colored granules, are extensively used in roll roofing and asphalt shingle compositions. The roofing granules are embedded in the asphalt coating on the surface of the asphalt-impregnated felt base material, the granules thus forming a coating that provides an adherent, weather-resistant exterior roofing surface.
Mineral-surfaced asphalt shingle roofing can support the growth of discoloring algae, most commonly of the blue-green type (Cyanobacteria). Such roofs can develop spots of algae colonies within 2–3 years of exposure, particularly in the southeastern gulf states, as a result of inoculation by air-born desiccated cells. These spots gradually grow into unsightly streaks as rain washes cells down the roof. In severe cases, this discoloration will eventually overtake the entire roof.
In addition to being unsightly, algae discoloration reduces the reflectivity of light-colored asphalt shingles and thus increases their peak daytime temperatures. Some have argued that his can reduce the effective service life of the roof. Although algae discoloration can be removed by cleaning, this process is costly and will have to be repeated every few years. A more effective approach is to utilize algae-retardant roofing granules as a component of the asphalt shingles to prevent the growth of discoloring algae in the first place.
Illustrative examples of prior art compositions used in concert with roof shingles are as follows.
U.S. Pat. No. 2,732,311 discloses the use of metallic flakes, such as aluminum, copper and bronze flakes to produce radiation-reflective roofing granules.
U.S. Pat. No. 3,894,877 discloses incorporating copper silicate into color coated roofing granules using heavy processing oil to have the copper silicate adsorbed into the color coat.
U.S. Pat. No. 4,092,441 discloses roofing granule treatment by coating the roofing granules with metallic algaecides such as zinc, copper, nickel and mixtures thereof which are sprayed in the form of molten droplets onto the surface of the roofing granules or onto the surface of asphalt roofing.
U.S. Pat. No. 4,378,403 discloses roofing granules coated with insolubilized reaction product of a coating compositions comprising water, kaolin clay, sodium silicate, pigment, and gas-forming compounds. The gas forming compound includes hydrogen peroxide, alkali metal perborates, alkali metal persulfates, alkali metal borohydrides, and alkali metal azides, and are used for the purpose of enhancing the opacity of the coating.
U.S. Pat. No. 5,356,664 discloses a method of inhibiting algae growth on an asphalt shingle surface using a blend of copper-containing algae-resistant and non-algae-resistance granules.
U.S. Pat. No. 5,411,803 discloses three-layer coated ceramic granules. The ceramic granules comprise the reaction product of an alkali metal silicate and aluminum silicate. The ceramic coating further includes a borate compound and zinc oxide.
U.S. Pat. No. 6,214,466 discloses algae-resistant roofing granules coated with: a first coat consisting of a fired silicate-clay matrix containing cuprous oxide and zinc sulfide to provide a slow, long-term bimetallic copper and zinc ions release; and a second coat consisting of a fired silicate-clay matrix containing a pigment.
Algae-retardant granules currently available include those in which a substantial loading of cuprous oxide (by itself or in combination with zinc compound) is incorporated in some of the semi-ceramic coatings that encapsulate a crushed rock base. At least two (2), and sometimes three (3), ceramic coatings are used in which the cuprous/zinc compounds are incorporated in the inner coating(s) and inorganic pigments, which determine the overall product color, are incorporated in the outer coating. These products are designed to be blended with standard granules at a 10–15% rate and to provide a continuous release of algicidal copper/zinc inons in the presence of moisture from rain and dew. However, the rate of copper/zinc release is often insufficient despite the high loading of cuprous/zinc compounds, due to low porosity of the outer coating, which acts as a barrier to copper/zinc ion migration. This can result in premature failure of the algae-retardant granules and the appearance of unsightly discoloration.